Soybean pod drop causes concern

David Lanclos | Sep 25, 2002

To digress some, early yield reports indicate that Group IV beans have yielded exceptionally well. Reports on Group IV beans that I received last week were 35 to 63 bushels per acre. In addition to that, most of the Group V and VI beans that I have seen throughout the state are excellent and should yield very well. We have the potential to have one of the best soybean crops that Louisiana has had in several years.

Although the crop is in good shape, disease and insect pressure has been extremely high. We have had to treat for insects twice in many situations, especially for stinkbugs and loopers.

Dr. Ken Whitam, LSU AgCenter Extension Pathologist, told me last week that he estimated that 30 percent of the soybean acres across the state (mainly in the south) had at least one fungicide application and in some cases two. We have been fortunate that our later planted beans had optimal growing conditions and thrived but so have the insects and diseases.

Pod dropping is not an unfamiliar sight in soybean fields, but it can be alarming when, for no apparent reason, plants start to drop pods. According to Dr. Jim Board, LSU soybean physiologist, there are a couple of reasons as to why this phenomenon occurs.

“The soybean plant always sets more pods than it intends to keep, even under the most optimal of growing conditions, so the distal pods will normally abort when they are small (less than 1 inch in length) and the proximal pods, which already at full length have small seeds, will be held by the plant and go on to production,” he says.

“Adverse environmental conditions (drought, extreme heat, waterlogging, inadequate light interception, etc.) will simply increase the number of small pods that abort during the R3 to R6 period (i.e. pod formation period). By the time the crop reaches R6 (about 8 to 12 days after R5), final pod and seed number for the crop is usually determined, and optimizing yield is just a matter of making sure seed filling is maximized. After R6, even the most severe environmental stresses will rarely cause pod abortion. Instead, the crop will respond to these stresses by reducing seed weight, but keeping the same number of pods.”

It is very common to blame pod drop on “some environmental stress,” which is an inaccurate assessment, according to Dr. Board. “If there has been a lack of rain, it is common to call it drought stress or if temperatures were high, pod drop is often attributed to heat stress. Drought stress during rapid seed fill (R6 to R7) does not cause pod abortion but reduces seed size and there is ample research that demonstrates this. In addition to this research, temperatures in Louisiana rarely get into the range where heat stress could occur (105 to 110F).”

According to information from the North Carolina Extension Service, a typical soybean crop will produce more flowers than pods, and yield is relatively unaffected by which flowers become pods. The number of pods carried to maturity will primarily be a result of the amount of photosynthate available during pod-set. If more pods are present than the supply of photosynthate can support, the plant will abort (or shed) some pods. If fewer pods are available than the photosynthate will support, the plant will continue flowering for as much as two to three weeks longer than normal and will set additional pods. There are numerous fields that have experienced severe pod drop that are starting to flower and reset new pods.

Diseases and insects can play a large role in soybean pod drop. According to Dr. Whitam, heavy infestations of aerial blight, such as we have had this year, can cause pod drop by attacking the plant during flowering or early pod set. Dr. Whitam also mentioned that, in some cases, if aerial blight is allowed in the middle of the canopy, severe pod drop can occur.

According to Dr. Jack Baldwin, LSU Extension entomologist, “stinkbugs can cause pod drop in circumstances where heavy infestations are present during pod formation.”

Soybeans will continue to drop pods due to a number of factors that for the most part, we have some control over. During seed fill, we need to do as much as possible to avoid unnecessary stresses, such as insects or diseases.

In some of the cases that I observed, it was “natural” pod shed that the plants were going through because of the optimal growing conditions that we are experiencing however, in some cases insects predominantly stinkbugs or aerial blight played a part in the pod drop also.

David Lanclos is the Extension grain specialist with the LSU AgCenter. He is based at the Dean Lee Research Station, Alexandria, La.